Make calls to cmsis-os to get thread state, stack size, and max stack
usage rather than accessing internal RTX data directly. Wrap RTX5
specific code in OS_BACKEND_RTX5.

Also refactor the code to use mbed types rather than RTX types:
os_timer_t -> mbed_rtos_storage_timer_t
os_event_flags_t -> mbed_rtos_storage_event_flags_t
osRtxMutex_t -> mbed_rtos_storage_thread_t